1. Field of the Invention
This invention relates generally to plasma enhanced chemical vapor deposition (PECVD) reactors, and more particularly to a method and arrangement for depositing film on a semiconductor wafer in a PECVD reactor.
2. Description of the Related Art
Dielectric film deposition has been used extensively during the processing of semiconductor wafers. Several types of silicon-based plasma dielectric films are currently deposited on the surface of wafers during wafer fabrication, including silicon oxide, silicon nitride, silicon oxynitride, phosphorous-doped silicate glass (PSG), and boron/phosphorous-doped silicate glass (BPSG). Of these films, silicon dioxide (SiO.sub.2) deposited by chemical vapor deposition (CVD) has been found to be particularly useful in wafer formation.
CVD is the formation of a nonvolatile solid film on a substrate by the reaction of vapor-phase chemicals (reactants). CVD is a material synthesis process in which constituents in the vapor-phase chemicals react chemically near or on a wafer surface to form a solid product. Several steps occur during CVD reactions. These steps include the transport of reacting gaseous species to the wafer surface, the absorption of the species on the wafer surface, a heterogenous surface reaction catalyzed by the wafer surface, the desorption of gaseous reaction products, and the transport of the reaction products away from the wafer surface. CVD is a favored deposition process in many respects, primarily because of its ability to provide highly uniform films on wafers. Uniformity is defined as the consistency in the thickness of films deposited on wafers.
Numerous CVD deposition methods are well known, including atmospheric-pressure CVD (APCVD), low-pressure CVD (LPCVD), and PECVD. Rather than relying solely on thermal energy to initiate and sustain chemical reactions, PECVD uses a radio frequency (rf) induced glow discharge to transfer energy into reactant gases. The plasma-inducing glow discharge is generated by the application of an rf field to a low pressure gas, thereby creating free electrons within the discharge region. The electrons gain sufficient energy from the electric field so that when the electrons collide with gas molecules, gas-phase dissociation and ionization of the reactant gases occurs.
Advantageously, the PECVD process can take place at lower temperatures than can APCVD and LPCVD processes. This ability to process wafers at lower temperatures is a major advantage of PECVD because it provides a method of depositing films on some wafers which do not have the thermal stability to accept film deposition by other methods. In addition, PECVD can enhance the deposition rate when compared to thermal reactions alone, and can produce films of unique composition and properties.
As noted above, it is important for a PECVD reactor to have the ability to form films on wafers that have consistent thickness. One technique for obtaining a particular film thickness involves placing a disposable sample in a PECVD reactor for a fixed period of time, and depositing a film according to a PECVD process. Subsequently, the thickness of the film across the surface of the sample is measured. These steps are repeated several times so that the deposition rate for the reactor can be determined. Afterwards, a film is deposited on a wafer for a period of time corresponding to the period of time expected to achieve the desired film thickness on the wafer based upon the previously determined deposition rate.
One of the main problems with forming wafer films using PECVD reactors is parameter control. It is generally difficult to identify the effects of a specific process parameter because the PECVD system is complex. For example, plasma rf power and frequency, and reactant flow rates, concentration, and temperature all interact with each other, making it difficult to optimize a PECVD process. There are three main factors to consider when optimizing a PECVD process. First, maintaining plasma stability is important. To maintain a stable plasma, the plasma should be confined between the electrodes in the PECVD reactor and kept away from the walls of the reactor chamber. Second, the plasma density should be optimized for the desired deposition rate and uniformity. This optimization can be accomplished by adjusting the rf power and pressure, and the reactant gas flow rates. Finally, wafer temperature and position in a plasma should be tightly controlled. Differences in film thickness and composition can occur depending on variations in temperature and wafer position.
PECVD reactors have a structure which supports the wafer during the PECVD process. This wafer support, commonly referred to as a susceptor or a chuck, can be composed of any suitable supporting material. Most frequently, however, the wafer support is composed of a metal such as aluminum. Aluminum wafer supports are composed of a plate and a ceramic webbing attached to a portion of the plate. During the PECVD process, the plate tends to warp and the ceramic webbing tends to detach from the plate. As a result, aluminum wafer supports routinely need to be replaced. Unfortunately, replacement of the aluminum wafer support is time-consuming, taking an average of six hours to complete.
Throughput can often be increased by installing a ceramic wafer support in a PECVD reactor, rather than an aluminum wafer support. A ceramic wafer support is commonly composed only of a plate, and therefore does not have a webbing which tends to detach from the plate during PECVD processing.
Additionally, a ceramic wafer support does not have a tendency to warp.
Consequently, ceramic wafer supports are often more cost-effective than aluminum wafer supports.
As noted above, however, altering a parameter of a PECVD process may impact the uniformity of the film deposited on the surfaces of wafers during the PECVD process. Accordingly, using a ceramic wafer support rather than an aluminum wafer support can reduce process uniformity, especially when the replacement occurs in a reactor which is configured for an aluminum wafer support. Numerous steps can be taken to counter any such reduction in process uniformity, including adjusting the position of the wafer within the reactor during the PECVD process.